Re: [RFC][Patch v1 3/3] PCI: Limit pci_alloc_irq_vectors as per housekeeping CPUs

Marcelo Tosatti
2020-09-10T19:23:26Z

On Wed, Sep 09, 2020 at 11:08:18AM -0400, Nitesh Narayan Lal wrote:
> This patch limits the pci_alloc_irq_vectors max vectors that is passed on
> by the caller based on the available housekeeping CPUs by only using the
> minimum of the two.
>
> A minimum of the max_vecs passed and available housekeeping CPUs is
> derived to ensure that we don't create excess vectors which can be
> problematic specifically in an RT environment. This is because for an RT
> environment unwanted IRQs are moved to the housekeeping CPUs from
> isolated CPUs to keep the latency overhead to a minimum. If the number of
> housekeeping CPUs are significantly lower than that of the isolated CPUs
> we can run into failures while moving these IRQs to housekeeping due to
> per CPU vector limit.
>
> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
> ---
> include/linux/pci.h | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 835530605c0d..750ba927d963 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -38,6 +38,7 @@
> #include <linux/interrupt.h>
> #include <linux/io.h>
> #include <linux/resource_ext.h>
> +#include <linux/sched/isolation.h>
> #include <uapi/linux/pci.h>
>
> #include <linux/pci_ids.h>
> @@ -1797,6 +1798,21 @@ static inline int
> pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
> unsigned int max_vecs, unsigned int flags)
> {
> + unsigned int num_housekeeping = num_housekeeping_cpus();
> + unsigned int num_online = num_online_cpus();
> +
> + /*
> + * Try to be conservative and at max only ask for the same number of
> + * vectors as there are housekeeping CPUs. However, skip any
> + * modification to the of max vectors in two conditions:
> + * 1. If the min_vecs requested are higher than that of the
> + * housekeeping CPUs as we don't want to prevent the initialization
> + * of a device.
> + * 2. If there are no isolated CPUs as in this case the driver should
> + * already have taken online CPUs into consideration.
> + */
> + if (min_vecs < num_housekeeping && num_housekeeping != num_online)
> + max_vecs = min_t(int, max_vecs, num_housekeeping);
> return pci_alloc_irq_vectors_affinity(dev, min_vecs, max_vecs, flags,
> NULL);
> }

If min_vecs > num_housekeeping, for example:

/* PCI MSI/MSIx support */
#define XGBE_MSI_BASE_COUNT 4
#define XGBE_MSI_MIN_COUNT (XGBE_MSI_BASE_COUNT + 1)

Then the protection fails.

How about reducing max_vecs down to min_vecs, if min_vecs >
num_housekeeping ?